The invention relates generally to printing liquid droplet ejectors such as used in ink jet printers, and in particular to an ejector in which a printing liquid is ejected from an opening in a nozzle as at least one droplet.
Ink droplet ejectors for use in ink jet printers are generally know. Printheads employing thermal bubble jet and piezo droplet ejectors to eject successive droplets of ink from a droplet ejection opening have found substantial commercial success. Recently, other classes of ink droplet ejectors have been become known, including those based on the motion of a mechanical piston or diaphragm. Known ejectors of this type include a cavity for containing the ink, a nozzle plate having a droplet ejection opening at the cavity, and a piston or diaphragm which can be moved or translated in the cavity to eject successive droplets of the ink from the opening. The motive force translating the piston or diaphragm is typically provided from thermal bimorphs, piezo-electric bimorphs, electrostatic membranes, magnets, etc.
For example, prior art U.S. Pat. No. 5,644,341 issued Jul. 1, 1997 discloses that the diaphragm is first distorted and then relaxed, by means of an electrostatic charge having successive different voltages, to eject the ink droplets. Also, see prior art U.S. Pat. No. 6,357,865 B1 issued Mar. 19, 2002.
Instead of a diaphragm, prior art U.S. Pat. No. 6,318,841 B1 issued Nov. 20, 2001 discloses that a so-called xe2x80x9cpiston layerxe2x80x9d can be used, which is moved towards the opening in the nozzle plate to eject the ink droplets when an electrical field is applied between the piston layer and the nozzle plate.
Another substitute for the diaphragm is disclosed in prior art U.S. Pat. No. 6,234,609 B1 issued May 22, 2001. In this instance, a so-called xe2x80x9cejection paddlexe2x80x9d is translated mechanically or pivoted towards the opening in the nozzle to eject the ink droplets when a thermal actuator for the ejection paddle is pivoted away from the opening. The motive force translating or pivoting the xe2x80x9cejection paddlexe2x80x9d is provided by a thermal bimorph.
In all cases, it appears that the ink is ejected from the droplet ejection opening when the diaphragm, piston or the like is actuated within the cavity. Moreover, prior to the next actuation, the ink refills the cavity via a connection to an ink reservoir.
An important parameter regarding the actuation is the volumetric efficiency of the printhead, as discussed by Gooray et.al. in the Journal of Imaging Science and Technology, Vol. 46, No. 5, published September/October 2002. The volumetric efficiency characterizes the ratio of the volume of liquid ejected to the volume of liquid returned to the reservoir, or, for droplet ejectors in which a diaphragm is moved, the volumetric efficiency also characterizes the ratio of the volume of liquid ejected to the volume swept out by the motion of the diaphragm. If the volumetric efficiency is low, the energy required to eject a droplet is large, leading to an excessive generation of heat. Additionally, if the volumetric efficiency is low, the volume of the ejected droplet and the velocity of the ejected droplet may be reduced, all of which are well known in the art of inkjet printing to be undesirable.
Prior art U.S. Pat. No. 6,102,530, issued Aug. 15, 2000, discloses multiple thermal means for heating ink near the ejection opening prior to droplet ejection. Such heating generally increases volumetric efficiency, since the heated liquid near the ejection opening flows more readily and since a secondary bubble occludes a refill channel during drop ejection. However, additional heating pulses consume power. Prior art U.S. Pat. No. 5,880,752, issued Mar. 9, 1999, discloses active valves, for example bimetallic valves, separating the ink cavity from the reservoir. When such valves are closed during droplet ejection, the volumetric efficiency is increased. However, the complexity of building valves increases the cost of droplet ejectors.
For the case of a xe2x80x9cpiston layer,xe2x80x9d as described in the above-mentioned prior art U.S. Pat. No. 6,318,841 B1, the volumetric efficiency may be controlled by locating the edge of the piston layer precisely in relation to the inner surface of the ink cavity. If the edge and the surface are closely located, the connection between the ink cavity and the reservoir impedes the ink flow, and the volumetric efficiency is large. However, since the ink cavity must be refilled through the connection, the actuation frequency of the droplet ejector will be small, which reduces printing productivity, as is well known in the art. Thus, the volumetric efficiency must be compromised to maintain a high actuation frequency.
In light of the above, it is desirable to provide a droplet ejector with increased volumetric efficiency without increasing its cost and without decreasing its frequency of actuation. This is believed to be accomplished by the invention, in several embodiments to be described.
According to one aspect of the invention, a printing liquid droplet ejector, comprises:
a cavity for containing a printing liquid;
a nozzle having a droplet ejection opening at the cavity;
a liquid holding unit in the cavity having a volume sufficient to hold some of the printing liquid in the cavity, being mechanically translatable toward the opening, and being volumetrically alterable to reduce its volume to cause at least some of the printing liquid held by the unit to be expelled from the unit to in turn cause either printing liquid expelled from the unit or other printing liquid in the cavity to be ejected from the opening as at least one droplet when the unit is mechanically translated toward the opening; and
a force applying device for applying a motive force to the liquid holding unit to mechanically translate the unit towards the opening and volumetrically alter the unit to reduce its volume.
According to another aspect of the invention, a method of ejecting printing liquid as one or more droplets using a liquid holding unit that is volumetrically alterable to reduce its volume to expel a printing liquid held by the unit, comprises:
placing a printing liquid into a cavity including a nozzle having a droplet ejection opening; and
applying a motive force to the liquid holding unit in the cavity to mechanically translate the unit towards the opening and, when the unit is at least close to the opening, to volumetrically alter the unit to reduce its volume so that either printing liquid expelled from the unit or other printing liquid in the cavity is ejected from the opening as at least one droplet.